This application relates generally to turbine engines, and more particularly, to structural shells used in axial flow gas turbine engine systems.
Axial flow gas turbine engines typically includes a plurality of second members, such as a fan rotor assembly, a booster assembly, a compressor, and a turbine. The fan rotor assembly includes a fan including an array of fan blades extending radially outward from a rotor shaft. The rotor shaft transfers power and rotary motion from the turbine to the compressor and the fan, and is supported longitudinally with a plurality of bearing assemblies. Bearing assemblies support the rotor shaft and typically include rolling elements located within an inner race and an outer race.
Structural casings extend around the turbomachinery such that radial clearances are defined therebetween. Inadequate clearances defined within the turbine engincs, such as, but not limited to clearances between rotating seals and stationary members, between bearing elements and bearing races, between a bearing race and a damper housing, and/or between rotor blades and surrounding casing, may adversely affect performance of the associated turbomachinery. Howevcr, maintaining control of such clearances may be difficult during engine operation as the second members may expcrience distortions which may alter the clearances defined betwcen the casings and second member. For example, in the case of a fan assembly, axial thrust generated by an engine may be reacted by a thrust links coupled between the fan assembly and the engine frame. The thrust links may cause the frame to ovalize into a lobed pattern, that may not attenuate through the engine structure, but rather may be propagated into the attaching structures forward and aft of the fan frame.
To facilitate maintaining substantially constant clearances during engine operation, at least some known high pressure compressor casings and bearing housings, such as are utilized on the GE 90-115 engine, have accommodated such thrust loading deflections by directly offset grinding the case or critical bores to an out-of-round condition (known as a pre-lobed condition) during assembly. The distortion due to thrust load essentially cancels the oval manufacturing shape, and causes the case bore to assume a substantially round condition at a pre-determined operating thrust point such that respective rotor-to-stator, and/or bearing, clearances are facilitated to be radially maintained. However, direct machining such components may be a time consuming process that may be repeated several times until the critical bore shape is obtained.